Most wastewater contains organic contaminants that fall into two groups; carbonaceous, which is derived from sugars, starches and other carbohydrates, and nitrogenous, which comes from the degradation of more complex compounds, such as proteins and amino acids, into ammonia. Most conventional treatment systems address these contaminants separately by using different species of aerobic microorganisms which can use the contaminants as food source. Generally, naturally occurring microorganisms are used and maintained in an environment that keeps them in intimate contact with the food source and provides them with enough oxygen to support their metabolism as they reduce or oxidize the contaminants.
Most conventional biological treatment plants use an activated sludge process to accomplish this oxidation and reduction of the organic material. The conventional activated sludge process and its many variations has been the industry standard for over fifty years. A variable depth reactor is generally used to operate a variation of the activated sludge process to biologically convert carbonaceous biological oxygen demand (CBOD) into carbon dioxide, water, and biological cell mass. This conversion happens rapidly as the sugars and starches present in the wastewater are absorbed by the microorganisms in the activated sludge.
The biological oxidation of nitrogen compounds, specifically ammonia, into nitrates is accomplished by a different group of microorganisms that do not become the predominant species until the easier to consume food (CBOD) has been reduced. Therefore, this nitrification step takes place as a secondary reaction, but only after the bulk of the CBOD has been reduced. Generally, the nitrification step takes place either separately in a second reactor, or in conjunction with the CBOD reduction in a much larger single reactor. The nitrate generated in the nitrification step has been found to have a detrimental impact, in particular on aqueous environments. Therefore, although many existing municipal regulations require only CBOD reduction and ammonia oxidation for wastewater treatment, growing environmental concerns have led to increasing demands for further treatment of the nitrified wastewater. Known de-nitrification processes use yet another set of different, anoxic microorganism, which are able to pull oxygen from the nitrate (NO3) and liberate nitrogen gas into the atmosphere. This removes the nitrogen based nutrients from the wastewater. However, the de-nitrification process is complicated and requires the addition of a carbon based food source to satisfy the metabolic needs of the anoxic microorganisms as they break the nitrate.
In conventional systems, treatment of the wastewater to reduce the CBOD and nitrogen based nutrients requires the use of four treatment vessels, an aeration basin for the CBOD reduction process, a nitrification basin, an anoxic de-nitrification basin and an aerobic basin. The hydraulic capacities of the aeration, nitrification, anoxic de-nitrification and aerobic basins is commonly 1×, 0.25×, 0.25× and 0.25×. If the CBOD reduction and nitrification steps are carried out in the same reactor, the hydraulic capacity of the required reactor basins is 1.5×, 0.25× and 0.25×. Consequently, the nitrification and de-nitrification process adds significantly to the complexity and cost of the overall system.